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Share Capital - Changes in Number of Shares of Class A Stock Outstanding (Details) - Class A Stock - shares
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Class A Stock outstanding, beginning of period 12,592,646 12,445,479 12,381,778 12,698,703
Issued pursuant to shared-based compensation plans 31,582 43,238 242,450 390,562
Repurchased and canceled pursuant to the stock buy-back 108,494 84,290 108,494 684,838
Class A Stock outstanding, end of period 12,515,734 12,404,427 12,515,734 12,404,427